In external drum imaging systems, a movable optical carriage is commonly used to displace an image recording source in a slow scan direction while a cylindrical drum, with recording media mounted on an external surface thereof, is rotated along its longitudinal axis. The drum rotation causes the recording media to advance past the image recording source along a fast scan direction that is substantially perpendicular to the slow scan direction.
The image recording source may include an optical system for generating one or more imaging beams that are scanned across the surface of the recording media. Each imaging beam may be separately modulated according to a digital information signal representing data corresponding to the image to be recorded.
The recording media to be imaged by an external drum imaging system is commonly supplied in discrete, flexible sheets, hereinafter collectively referred to as “printing plates.” Each printing plate may include one or more layers supported by a support substrate, which for many printing plates is a plano-graphic aluminum sheet. Other layers may include one or more image recording (i.e., “imageable”) layers such as a photosensitive, radiation sensitive, or thermally sensitive layer, or other chemically or physically alterable layers. Printing plates that are supported by a polyester support substrate are also known and can be used in the present invention. Printing plates are available in a wide variety of sizes, typically ranging, e.g., from 9″×12″, or smaller, to 58″×80″, or larger.
Many clamping systems are available for holding an end or edge of a printing plate on the external drum of an imaging system (i.e. on the external surface of the drum). One example of a leading edge clamping mechanism is disclosed in U.S. Pat. No. 6,412,413 granted on Jul. 2, 2002 to Tice et al., herein incorporated by reference in its entirety for background information only. Although the leading edge clamping mechanism disclosed in this reference is highly effective, it has several drawbacks. For example, the leading edge clamping mechanism uses a fixed actuator to selectively open and close a clamping portion. As such, the clamping portion can only be opened and closed when it is positioned at a single circumferential position of the external drum (i.e., under the fixed actuator). Further, the leading edge clamping mechanism is a fairly complex mechanical assembly that has a high part count and is expensive to produce.
Another clamping mechanism is disclosed in U.S. Pat. No. 6,705,226 granted to McManus on Mar. 16, 2004, herein incorporated by reference in its entirety for background information only. A mechanism for clamping a leading end of a printing plate on an external drum of an imaging system includes: a clamping portion disposed on the external drum, wherein the clamping portion may be selectively displaced between a clamping position for clamping the leading end of the printing plate against the external drum and a release position; and an inflatable member positioned between the clamping portion and the external drum for displacing the clamping portion between the clamping position and the release position.
One major drawback of the '226 patent is movement of the printing plate during clamping. Specifically, when the bladder is deflated to cause the distal end of the clamping portion of the clamping apparatus to clamp down on the plate, the distal end moves slightly in a direction away from the registration pins, in turn causing the plate to move slightly in the direction away from the registration pins. This movement results in misalignment of the plate on the external drum.